1. Field of Invention
The present invention concerns an instrument for measuring the refractive index of a fluid, in particular of seawater.
2. Description of Related Art
At present, calculations of the physical properties of seawater are accomplished based on measurements of three variables: pressure, temperature, and conductivity. Simultaneous measurement of pressure (P), temperature (T), and conductivity (C) makes it possible to calculate salinity (S) on the basis of an international scale (Practical Salinity Scale, 1978 [PSS-78]). If the values of parameters S, T, and P are known, it is possible to calculate the specific gravity of the water (.rho.), its specific volume (V=1/.rho.), and its specific gravity discrepancy (.gamma.=.rho.-1000 kg/m.sup.3). The value of V is then used to calculate depth and, in particular, the speed of sound.
Although measurements of P and T can be performed with sensors whose accuracy and stability arc sufficient thanks to periodic recalibration, the same is not true for the parameter C, which is measured using cells which are sensitive to marine pollution. In addition, calibration of such cells is still difficult to perform. Since T and C must be measured simultaneously, response time adjustment problems may also impair the accuracy with which S can be calculated. It must also be noted that salinity is a parameter which accounts for less than 20% of any change in conductivity, and that salinity is defined in PSS-78 based on the conductivity ratio of a KCl solution and not on the basis of the conductivity ratio of a reference seawater, since the latter cannot be measured directly. The result is errors on the order of several tens of ppm in estimating the specific gravity.
Another method, known for about a hundred years, does exist for estimating directly the salinity and in particularly the specific volume of a substance. This method requires a measurement of the local, in-situ value of the optical refractive index (n). The Lorentz-Lorentz equation yields a value for n directly as a function of the specific gravity of a substance, to within 3%. Achieving greater accuracy requires calculating a polynomial which relates n to T, P, and S at a given wavelength.
The value of n varies sensitively as a function of four parameters: wavelength (.lambda.), temperature (T), pressure (P), and concentration of solutes (NaCl, KCl, etc.), which may be called salinity (S). It is therefore necessary to know accurately the equation(s) which relate(s) these four parameters to the refractive index in order to understand how it is affected by changes in each one. Millard and Seaver have established equations which relate the index to the temperature, pressure, salinity, and density of seawater. They have shown that the polynomial equation which relates n to density is simpler than the one based on a measurement of C. This equation is at the moment less accurate, but more reliable.
The refractive index of liquids and gases is generally measured with reference to the index for air, which is known to an accuracy better than 5.times.10.sup.-8 by way of the Bengt-Edlen equation. Laboratory measurements are performed using optical interferometers of Mach-Zehnder design (a variant of the Michelson interferometer), or Fabry-Perot design. Measurements in industrial or clinical contexts are also made by interferometry, using instruments that are less accurate but are portable. They can also be made using fiber-optic instruments based directly on a measurement of refractive angles.
In the laboratory, the salt concentration of seawater is measured using instruments called salinometers, which measure the conductivity of the water in question with reference to the conductivity of a reference water. While the quality of reference waters is beyond question, there can be variations between one batch and another, which impairs the reliability of the results.
Precision refractometers have been produced for making measurements at sea. U.S. Pat. No. 4,699,951, for example, describes an original method using a refractometer/salinometer which can be used at oceanographic anchorage sites, and is based on measuring the extinction of wavelengths by total reflection. Also known, from an article by Mahrt and Waldmann, is a densitometer based on a refractometry principle with which microdensity profiles can be performed very quickly to an accuracy of 1.times.10.sup.-6 for n, which represents a relative uncertainty of 0.0017 kg/m.sup.3 for the density. A Russian team has also created a device with which n can be measured to a relative accuracy of 1.times.10.sup.-6, but this device, which is a Mach-Zehnder interferometer, is extremely bulky.
Thus, although the capabilities of these devices are interesting, the fact remains that they are large instruments which do not yield absolute measurements of refractive indices.